In the production of yeast fermented beverages, continuous fermentation offers a number of significant advantages, including:                higher productivity and lower investment: equipment can be operated for prolonged periods of time under full load, meaning that for equal production volume smaller vessels are needed than in a batch process;        constant and better quality: process is easier to control due to possibility of adapting process parameters to local and instantaneous requirements and because steady-state-conditions are much more stable;        high hygienic standard: continuous process is operated in a closed system.        less energy: energy consumption is evenly spread, without major use peaks; and        less labour: operation of continuous process requires less attention        less standstill and cleaning: continuous process can be operated at much longer runlengths than batch processes.        
These potential advantages of continuous fermentation have been recognised by the brewing industry a long time ago. Accordingly, many attempts have been made to design continuous fermentation processes that do indeed deliver those potential benefits.
British patent GB-B 926 906 describes a process for the production of potable beer that can be operated continuously, which process comprises supplying a stream of unfermented wort to an enclosed vessel containing a vigorously agitated, substantially homogeneous, body of fermenting wort and yeast held at a substantially constant temperature, the rate at which the unfermented wort is added being arranged to displace fermented wort therefrom at a desired degree of attenuation of fermentable sugars, the displaced fermented wort being led from the enclosed vessel through an upwardly directed settling chamber, within which the flow rate of the fermented wort is sufficiently slow as to permit a substantial proportion of the yeast to separate out and fall back into the enclosed vessel. The process is said to offer the advantage that it permits a more compact apparatus to be used as no separate yeast separation stage is required. Furthermore, it is observed in the British patent that with different strains of yeast the amounts of yeast present in the apparatus varied between 35 and 100 gms/liter.
U.S. Pat. No. 3,234,026 describes a method for the continuous fermentation of brewery wort for the production of a potable, non-distilled alcoholic malt beverage product characterized by passing said wort through a system of interconnected zones, continuously introducing wort to the first zone of said system and promoting rapid fermentation thereof by maintaining a yeast concentration in said wort above 12 grams per liter, maintaining said wort at a temperature above 48° F., maintaining said yeast in suspension in said fermenting wort by continuous agitation whereby to promote such rapid fermentation, thereafter separating yeast therefrom and continuously drawing off the substantially clarified fermented wort from said system at the same rate as the rate of introduction of the wort to the first of said zones. It is observed in U.S. Pat. No. 3,234,026 that when insufficient yeast is being propagated to maintain desired concentration the invention contemplates re-using a portion of the separated yeast in the system for this purpose. Furthermore, the US patent describes embodiments of the aforementioned method in which yeast propagation and product formation occur in different zones or vessels. In claim 29 an embodiment is described wherein the yeast level in the product formation stage is maintained above 12 and up to 100 grams with a moisture content of 80% for each liter of fermenting wort (equivalent to a wet yeast content of 9-74 g/l).
CA 545,867 describes a method for the continuous fermentation of a yeast fermentable substrate, said method comprising maintaining separate yeast propagation and product formation stages, maintaining aerobic conditions, favourable to yeast propagation in the yeast propagation stage; maintaining anaerobic conditions favourable for alcohol production in the product formation stage; continuously introducing fermentable substrate to the yeast propagation stage, continuously passing effluent from the yeast propagation stage to the product formation stage; separating yeast from effluent of the product formation stage and re-introducing controlled amounts of the separated yeast to the product formation stage to maintain the yeast concentration therein at a controlled level. The Canadian patent application teaches to maintain the yeast concentration in the product formation stage at 5% to 15% pressed cake. In the typical operation that is described in the Canadian patent application the yeast concentration in the propagation is stage is about 1.2% pressed yeast and the yeast concentration in the product formation stage is maintained constant at 10% by recycling as much yeast as may be necessary to achieve this concentration.